Current wind turbines are producing increasingly more power. This increase in power production is especially more prevalent in direct-drive wind turbines, that is, those using permanent magnets and with no gearbox, involving extremely large generator structures that are required for such amounts of power.
Large generators are very difficult to handle due to their very big size and weight. Furthermore, permanent magnet generators suffer from the problem of handling of active parts. Active parts include the permanent magnets and coil windings. Permanent magnets, which are fitted in the rotor of the generator, remain always active with no need for an excitation current. A magnetic field is thus always present, which renders handling of active parts very complex, both during installation and replacement operations.
In order to ease transportation in such wind turbines, modular technology is now being applied to at least some parts of the generator. The modular technology has been shown to be a good solution to reduce the problem of logistics since with modular technology generators do not have to be transported as a whole.
In this respect, segmented wind turbine stators have been proposed as described in EP2164154. Segmented wind turbine rotors have been also provided as shown in US20110266909. Other examples, such as in US2009134627, provide directly driven generators for wind turbines comprising stators formed of stator segments and rotors formed of rotor segments.
Another approach is to provide generators where both the rotor and the stator are segmented. This is shown, for example, in US2012112466, which discloses a wind turbine generator comprising a rotor and a stator that are both divided into a plurality of segments. The rotor segments are fitted with permanent magnets while the stator segments are fitted with corresponding coils. When the generator is being produced, the stator segments and the rotor segments are joined together by means of connecting elements. Through such connecting elements, the rotor and the stator are spaced apart from each other via an air gap separating them.
Document WO2011031165 also refers to a wind turbine generator comprising stator and rotor segments. The stator segments include a stator housing adapted to be arranged to one side of a bearing unit. Inside the stator housing stator windings divided into segments can be arranged. The rotor segments include magnet supports for the fixation of magnets. The rotor segments with magnets are positioned in place with special tools to keep the right air gap relative to the stator windings inside each generator stator segment. The stator segments and the rotor segments are locked to each other by a preliminary locking system.
The above solutions are specifically directed to the above mentioned problem of how to ease the transportation of the generator to the site of erection. As stated, this is carried out presently by dividing the generator rotor and/or the generator stator in substantially similar segments.
However, no efficient and cost effective solutions are described to date in the prior art for facilitating operations of maintenance and repair of the generator, even in the event that the generator is not segmented, or segmented in large parts. The applicant has found that, in addition to transportation and logistics, other important technical problems need to be solved or at least partially reduced in wind turbine generators such as transportation and storage of active parts.